The ever increasing demand for transmission capacity and higher signal speeds is driving the development of advanced technologies that allow a better use of deployed resources as well as an increase in transport rates. Existing DWDM systems divide the C-band optical spectrum into discrete bands, spaced usually by 50 or 100 GHz, and standardized by the ITU (FIG. 1). A transponder provides an individual wavelength carrying a client demand (which might be Ethernet or OTN, and might have a payload of anything up to 100 Gb/s), which can be accommodated in just one of these bands. Some of the demands fit comfortably between the 50 GHz grid boundaries (like d1, d4 or d5), whereas others (like d2 or d8) are too broad. Optical filters, specified to the ITU fixed grid will impose large filtering penalties as shown in FIG. 1.
This means that the resulting DWDM network is somewhat inflexible to changes in bandwidth demand. Although further transponders can be installed to cope with additional demands, this is a very slow process that can typically take many weeks due to the great deal of manual processes involved, e.g. placing the order for a new transponder, conducting the necessary installation and provisioning, testing, etc. Moreover, future high bit rate transponders, such as 400 Gb/s and above, are expected to utilize wider bandwidths than the current 50 GHz grid. Therefore, current infrastructure would not be able to support them. Therefore, large bandwidth demands would have to be divided up so that they can be carried over the fixed grid.
Elastic Optical Networks make it possible to use the optical spectrum in a more flexible way (FIG. 2). For instance, variable-size spectrum slices can be defined and allocated depending on the bandwidth requirements of individual channels. The ITU-T has recognized the requirement for a spectrum allocation scheme that provides more flexibility than the conventional 50-GHz grid. Therefore, the revised G694.1 recommendation considers a flexible DWDM grid with 12.5-GHz frequency slot granularity and 6.25-GHz central frequency granularity [2]. The EON approach advocates the use of new building blocks for an extended flexibility on resource assignment (be it capacity or spectrum) and an optimized use of the network capacity.
The main blocks of EON are the flexi-grid ROADM and the Bandwidth Variable Transponder (BVT). Flexi-grid ROADMs can filter signals with a granularity of 12.5 GHz, instead of 50 GHz like in current WDM systems. On the other hand, BVTs can adjust their transmission rate to the actual traffic demand, by expanding or contracting the bandwidth of an optical path (i.e. varying the number of subcarriers) and by modifying the modulation format, as depicted in FIG. 3a. There have been several demonstrations of bitrate-variable transmitters where the number of subcarriers or the modulation format is adapted to achieve the desired bitrate and spectral efficiency [3, 4].
However, when a high-speed BVT is operated at lower than its maximum rate, e.g. due to required reach or impairments in the optical path, part of the BVT capacity is wasted. In order to address this issue the Sliceable BVT (SBVT) has been proposed [1]. A SVBT is able to allocate its capacity into one or several optical flows that are then transmitted to one or several destinations, as illustrated in FIG. 3b. Thus, when an SBVT is used to generate a low bit rate channel, its remaining capacity can also be exploited for transmitting other independent data flows.
From the point of view of higher layers, an SBVT may be viewed either as a high-capacity BVT or as a collection of multiple logically/virtually independent lower-capacity BVTs, depending on the mode of operation. Possible BVT and SBVT configurations utilizing Nyquist WDM are shown in FIG. 4 [1].
The BVT, shown in FIG. 4a, comprises multiple light sources, modulators, quasi-ideal optical filters and a coupler. Multiple light sources, with spacing very close to the Nyquist limit, are independently modulated, filtered and coupled together in order to generate a multicarrier super-channel. The super-channel bit rate and bandwidth can be tuned by changing the modulation format (and carrier spacing) or by turning off unused carriers. The same configuration can be used to construct a SBVT by making the light sources and optical filters tunable. Thus, one or more carriers can be selected and utilized for transmission towards different destinations using different spectral bands, as shown in FIG. 4b. The number of carriers used for each optical flow is determined by the required channel bit rate.
Based on commercially available technologies is possible to implement SBVT. FIG. 5 depicts an example for a 400 Gbps SBVT. Thanks to Photonic Integrated Circuits (PIC), it is possible to have multiple carriers in the same component. Modulation formats can be programmed externally for each of the carriers. This example is based on carriers of 100 Gbps, which are transmitted using DP-QPSK. Next section presents different proposals for interconnect routers and SBVTs.
Existing solutions are similar to the one presented in FIG. 5. Using this architecture, network operators can reduce the number of transponders in the network and, consequently, the number of IP cards that are required in the network deployment. Let us assume a node demand as the one presented in FIG. 6a. Using transponders of 40 G and 100 G, six transponders are required to cope with the demands in FIG. 6a. These are: 1×100 G transponder for 80 G demand, 1×100 G and 2×40 G transponders for 150 G demand and 2×40 G transponders for 70 G demands. FIG. 6b shows the number of transponders required for each demand. In the case of using SBVTs (FIG. 6c) one SBVT of 400 G is required to cope with the demands in FIG. 6a. As the minimum granularity of this transponder is 100 G, the first demand (80 G) consumes 100 G, the second (150 G) uses 200 G and the third one (70 G) occupies 100 G.
The main problem of current SBVTs, which configures just the optical connections, is that there is not a system to reuse the optical resources for multiple flows, thus reducing the optical pipes utilization. When there is a fixed capacity, over-provisioning is the only solution to cope with users demands (FIG. 7).
Furthermore, state of the art SBVT implementations (e.g. FIG. 5) present very coarse optical granularity (e.g. 100 Gbps per flow). According to it, the number and power consumption of SBVTs required for a given traffic demand could be very high.
These figures could be reduced by improving SBVT granularity per flow. Finer granularity could be achieved by using: 1) New optical transmission technologies (e.g. Nyquist DWDM) and very precise optical filters or 2) Multilayer control mechanisms enabling L2 traffic flows going to a given destination to be split into different VLANs which could route over different paths.
Present invention is focused on the second alternative. While first option requires new hardware development the second one could be implemented over state of the art transmission and switching technologies. In particular, present invention enhances S-BVT performance and efficiency by enabling load balancing between packet and optical switching layers.